Provide sparse solve() methods for <dgCMatrix> that work via LU decomp.
"Cheap" fix for the lu() -> sometimes/occasionally unsorted columns in resulting L / U matrix bug:
In C, have new Csparse_sort2(.) version of Csparse_validate() and in
R a .sortCsparse(.) using .Call(Csparse_sort, x) -- seems to work ok... but want tests and docu;
also can be quite a bit more efficient already in C; -- TODOs